The present invention relates to a method for preparing a barium titanate based powder by an oxalate process, the barium titanate based powder being used in various fields such as ferro-electric materials, ferro-piezoelectric materials and the like.
The barium titanate based powder is one of the most important constituents of the electronic ceramics as dielectric materials. For example, the barium titanate based powders are widely used as raw materials for multi-layer ceramic chip capacitors(MLCC), positive temperature coefficient thermistors, piezoelectric devices, and the like.
They are conventionally produced by the dry-process that is mixing the powders of constituent elements and heating the mixture to high temperatures to cause a solid-phase reaction. The so-obtained powder consists of aggregates of irregular morphology, and usually requires high sintering temperature to achieve the desired characteristics. As the need of small size and large capacitance has been growing for electronic parts such as MLCC etc., it becomes important to prepare homogeneous, fine, and narrow size-distributed powder. Therefore the barium titanate based powders have been produced by wet-process like hydrothermal synthesis, oxalate method, alkoxide method, and so on.
In the hydrothermal synthesizing method, the process is very complicated because additional equipments like autoclave have to be used, and the productivity is somewhat low, so the prices of the obtained powders are high. Also, in the case of alkoxide method, the starting materials are difficult to handle, and their prices are very high. For these reasons, the barium titanate based powders are mostly manufactured by the oxalate method at present. The powders which are manufactured by the oxalate method have high purity, good morphology and reproducibility compared with the powders by the dry-process and other wet-process.
The oxalate method was developed by Clabaugh [xe2x80x9cPreparation of Barium Titanyl Oxalate Tetrahydrate for Conversion to Barium Titanate of High Purityxe2x80x9d, Journal of Research of the National Bureau of Standards, vol. 56, No. 5, pp. 289-291, 1956], and currently has been applied commercially to manufacture the barium titanate based powders. FIG. 1 illustrates the manufacturing process for the oxalate method by Clabaugh.
As shown in FIG. 1, barium chloride and titanium chloride are mixed together at around 1:1 ratio, and this mixture is added into oxalic acid, so that barium titanyl oxalate [BaTiO(C2O4)2xc2x74H2O] (to be called BTO below) is precipitated. Then the BTO is well washed, filtered and thermally decomposed at a temperature of about 800xc2x0 C., thereby obtaining the barium titanate based powder.
However, hard aggregates between the particles are formed during the thermal decomposition(or calcination). The strong milling is needed to remove these hard aggregates. Because distribution of the particle size become very broad during this strong milling, the density may be reduced, and some dielectric properties are badly affected. Because the extremely fine particles are enormously produced during the strong milling, it is hard to disperse the powder for forming and abnormal grain growth can be observed during the sintering process.
In order to overcome the above described disadvantages, Hennings et al. disclosed a new method for manufacturing the barium titanate based powders in U.S. Pat. No. 5,009,876. In this method, the mixing sequence in the Clabaugh process is altered, in such a manner that an aqueous barium chloride solution is added to mixed aqueous solution of oxalic acid and TiOCl2 at about 55xc2x0 C. They obtain the barium titanate powder with primary particles of 0.2-0.5 xcexcm, and aggregate size of 3-30 xcexcm.
As another example which is similar to that of Hennings, there is U.S. Pat. No. 5,783,165 of Wilson et al., in which discloses a new method for manufacturing the barium titanate powders that the Ba source is replaced to barium carbonate.
As another examples, Yamamura et al., disclosed a method in which they use (ethanol solution instead of water, thereby obtaining fine precipitates [xe2x80x9cPreparation of Barium Titanate by Oxalate Method in Ethanol Solutionxe2x80x9d, Ceramic International, vol. 11, No. 1, pp. 17-22, 1985], and further, Cho et al., have tried to obtain fine barium titanate particles with the replacement of the aging time and solvent [xe2x80x9cParticle Size Control of Barium Titanate Prepared from Barium Titanyl Oxalatexe2x80x9d, Journal of the American Ceramic Society, vol. 80, No. 6, pp. 1599-1604, 1997].
In all the above described methods, however, the severe aggregation problem occurring in manufacturing BaTiO3 cannot be essentially solved yet. Particularly, if the organic solvent instead of water is used to control the precipitate size, they will have some problems concerning cost and environment.
The present invention is intended to overcome the above described disadvantages of the conventional techniques.
Therefore it is an object of the present invention to provide a barium titanate based powder of which milling is easy, morphology is spherical, less-aggregated and uniform, and finally dielectric properties are superior through the revised oxalate method in this invention.
In achieving the above object, the method for manufacturing a BaTiO3 powder using the revised oxalate method according to the present invention includes the steps of:
adding a mixture of an aqueous barium chloride solution and an aqueous titanium chloride solution to an aqueous oxalic acid solution thereby precipiting BTO;
separating the precipitated BTO;
crushing the BTO so as to prevent the BTO from being aggregated after a thermal decomposition process;
thermally decomposing the BTO to form a barium titanate powder; and
crushing the barium titanate powder.
In another aspect of the present invention, the method for manufacturing a perovskite-type barium titanate based powder by an revised oxalate method according to the present invention includes the steps of:
adding a mixture of an aqueous barium chloride solution and an aqueous titanium chloride solution to an aqueous oxalic acid solution thereby precipitating barium titanyl oxalate;
separating the precipitated barium titanyl oxalate;
adding additives to the barium titanyl oxalate to replacing Ba or Ti sites of the barium titanate based powder;
crushing the mixture of said precipitated barium titanyl oxalate and additives to prevent the barium titanyl oxalate from being aggregated after a thermal decomposition process;
thermally decomposing the mixture of said precipitated barium titanyl oxalate and additives to form a perovskite-type barium titanate based powder; and
crushing the perovskite-type barium titanate based powder.